The evolution of wind turbine drive systems

WORLDWIDE:A turbine drive system includes the shaft turned by the rotor, sometimes a gearbox, and at least one generator to convert the mechanical power in the turning shaft to electrical power for the grid system.

The wind industry's drive system of choice continues to be the conventional high-speed geared system, with a market share now estimated at over 80%. The remaining proportion of installed wind turbines with drive systems that fall outside this category are largely direct drive models from German maker Enercon and Chinese producer Goldwind. The combined market shares of low- and medium-speed geared and alternative drive systems is still negligible.

The Nordex N80, a high-speed geared drive train, designed in 2000

Within the high-speed turbine segment interest is growing in applying permanent magnet generators (PMGs). These drive systems can minimise nacelle mass and offer superior partial-load efficiency compared with the more common doubly-fed induction generators (DFIGs). The new Vestas 1.8/2MW GridStreamer series is a prime example.

The switch to PMG was prompted by the Danish firm's belief that this generator type, which is completely decoupled from the grid, will meet the more stringent grid requirements likely to be applied in the future, particularly in wind markets with a high wind-power penetration. Another feature of the 2MW GridStreamer is a new gearbox design from ZF Wind Power, a company with a history in automotives.

High speed, low weight

Distributed drive systems split and distribute the load across more than a single generator. They have been in use for decades by multiple suppliers, but they feature in few wind turbines currently in operation. The most common gearbox design of this type has two output shafts and two generators. However, in the early 1990s former NedWind of the Netherlands released a 1MW model with four gear-driven generators of 250kW each plus two smaller belt-driven generators. It also manufactured a 250kW model with a single generator and a 500kW turbine with two generators in an unusual strategy to standardise generator size.
US firm Clipper's 2.5MW Liberty model has four PMGs, and there were plans to develop a 10MW Britannia turbine along the same lines, but this did not happen. Two years ago, German Winergy introduced a patented Multi Duored gearbox with two generators, of which two prototypes were fitted last year in Bard's new 6.5MW offshore turbine model.

The gearbox design is scalable to 12MW. It achieves multi-load sharing by feeding low-speed input torque via a central gearwheel redistributed to eight smaller, much faster rotating identical gear sets. From there, it is further stepped-up and redistributed to four gear sets and finally to two high-speed output shafts. This modular design enables 360o rotation of the gearbox casing around its main axis allowing easy inspection and inboard component exchange, without requiring jack-up vessel support.

A hybrid design

In the second half of the 1990s, the first prominent example of a low-speed geared drive system was developed and patented. The Multibrid was part of an innovative offshore turbine design project by German engineering consultancy Aerodyn Energiesysteme. Called a hybrid — between high-speed geared and direct drive — the Multibrid was claimed to combine the reliability of modern direct-driven turbines with the compact size of traditional high-speed geared systems. It was also said to be cheaper to produce than an equivalent direct-drive model. The first 5MW Multibrid M5000 consists of a single-stage planetary gearbox, a low-speed — about 150 revolutions per minute (rpm) rated — PMG and single rotor bearing, incorporated into a compact structurally stiff cast support structure.

The prototype was installed in 2004 and today four units are operating onshore and six offshore. Current owner Areva Wind will this year deliver 40 M5000 turbines for Germany's Borkum West II project and, by next year, should have delivered another 80 to the German Global Tech I project.

Finnish firm Winwind is licensed to build Mulitbrid-based turbines of up to 3MW. It has built a 1MW version since 2001 and a larger 3MW model since 2004. Both are fitted with a PMG.

Medium-speed trend

Medium-speed drive systems continue to be in demand. These models usually incorporate a two-stage planetary gearbox and a PMG or other 400-600rpm rated generator type. The 7MW Vestas V164-7.0MW in development is unusual in that it will have a three-stage gearbox, increasing a rotor speed of 10-12rpm to 400rpm.

South Korean shipbuilder DSME is developing a similar-style 7MW medium-speed offshore turbine with comparable rotor size but featuring a two-stage gearbox. The technology and layout of both the DSME and Vestas drive systems resemble those of Gamesa's G128-4.5MW and G128-5.0MW sister turbine models, including a main shaft with two-bearings and a medium-voltage PMG.

Geared drive-system developer and supplier Winergy is at the testing stage with its new medium-speed HybridDrive design, comprising a compact two-stage gearbox with a flanged PMG introduced in 2011. The innovative drive solution focuses on geared offshore and onshore wind turbines up to 6-7MW power rating. A prototype will be incorporated into a new 3MW Fuhrländer turbine model planned for this year. The FL3000 — like an earlier 2.5MW FL2500 model — incorporates patented Larus Compact technology. This is a gearbox life-enhancing design that transmits "pure torque" and prevents harmful rotor bending moments entering the gearbox.

Two-stage medium-speed drivetrains with PMG and full-converter are an interesting development trend, says Yann Rageul, head of sales and strategic consulting Europe at UK driveline engineering Romax Technology. "For power ratings of approximately 6MW and above this configuration offers superior cost-of-energy performance compared with single-stage low-speed as well as direct-drive systems," he says. "One reason is related to the rare earth elements required. Because slower turning generators are bigger in volume, they require substantially larger magnet quantities. Prices of Neodymium, used for PMG magnets, have increased by a factor of ten between July 2009 and July 2011 and, even though they have again reduced slightly today, their cost level remains high, driving up costs for direct-drive machines especially."

Computer aided

Romax has introduced new drivetrain simulation and analysis software. Used alongside engineering expertise Romaxwind can support gearbox design by uncovering why specific wind-turbine failures have occurred.

Romax is working with the US National Renewable Energy Laboratory (NREL) on innovative drivetrain developments. These include a novel 5MW medium-speed gearbox driving a PMG, and development of an optimum gearbox and generator combination to cut capital input, replacement, overhaul and operation and maintenance costs. "We are developing a complete drive system including the bedplate," says Rageul. "Our main expertise is with the drivetrain, of which a structurally stiff yet lightweight bedplate is an essential part, and a crucial precondition for meeting gearbox design life." He points to a previous job where a client had asked for help with a serious gearbox issue, but the Romax system analysis was able to show it was the bedplate that required optimising instead.

Romax is also developing its own drivetrain platforms up to 6MW, looking at design and analysis of semi-integrated drive systems versus modular drivetrains that traditionally consist of individually sourced components. This means an increasing focus on drivetrain system behaviour, dynamics and total cost of energy very early in the concept design process.

While only time will tell what drivetrain technologies will take centre stage in the future, the numerous ongoing research and development, technology and product development projects offer both valuable building blocks and fresh insight into next-generation drive systems and possible future solutions to some of the challenges ahead.

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